This invention relates to a method for reducing the concentration of multivalent metal cations in a brine solution containing a metal chelating agent. In particular, this invention relates to a method for reducing the concentration of metal cations such as iron, chromium, and nickel in a brine solution derived from a condensation polymer manufacturing process and containing a water-soluble metal chelating agent such as sodium gluconate.
The manufacture of condensation polymers often produces a brine solution as a by-product. For example, a brine solution is produced in the manufacture of polycarbonate resins through the reaction of phosgene with at least one bisphenol compound in an organic solvent in the presence of aqueous sodium hydroxide. A common example is the reaction of bisphenol A with phosgene in dichloromethane in the presence of aqueous sodium hydroxide to produce bisphenol A polycarbonate and sodium chloride solution.
To reduce production costs and avoid environmental pollution, such brine solutions are often recycled to a chlor-alkali plant for electrolysis to produce chlorine gas, sodium hydroxide solution, and hydrogen gas. The electrolysis cells in such chlor-alkali plants frequently comprise an anode compartment and a cathode compartment with an appropriate separator in between the two compartments. The purpose of the separator is to separate the anolyte solution and catholyte solution within the electrolysis cell. The separator may be at least partially porous to water. The types of separators used in electrolysis cells include diaphragms and membranes.
During electrolysis cell operation the separator may gradually become plugged with solid material, retarding the passage of water and dissolved species from anolyte solution to catholyte solution. Separator plugging decreases the efficiency of cell operation and lowers the production rate of products arising from electrolysis. When plugging reaches a critical point, the separator must be replaced, often before its expected life-time is reached. To achieve most economical electrolysis cell operation, it is necessary that the cell separator have as long a life-time as possible.
Brine solutions arising as by-products from condensation polymer manufacture often contain both organic and inorganic contaminants. Organic contaminants may include residual solvent, catalyst, and aqueous-soluble organic species such as monomer and low molecular weight oligomer. Inorganic contaminants may include multivalent alkaline earth and transition metal cations, particularly iron. When brine solution containing one or more such contaminants is electrolyzed, both organic species and metal species may precipitate on the surface of and within an electrolysis cell separator to cause plugging. To achieve maximum life-time of a separator in an electrolysis cell, the concentration of contaminating organic species and multivalent metal cations must be reduced to as low a level as economically possible in the feed-brine solution.
One method for lowering the concentration of organic and inorganic contaminants is known as primary brine treatment. In primary brine treatment, the brine pH is elevated to above about 10 in the presence of a molar excess of carbonate ion in order to precipitate alkaline earth and transition metals as their carbonates and/or hydroxides, followed by a filtering or settling process such as clarification. This is followed by acidification and stripping of the brine to remove carbonate ion as well as organic contaminants such as organic solvents and dissolved catalysts. Additional treatment such as adsorption may be utilized as necessary to remove organic species such as monomer and low molecular weight oligomer from the brine.
This primary brine treatment procedure may be effective for precipitating calcium, magnesium, and iron cations, and substantially reducing their concentration in brine solution as well as reducing the concentration of dissolved organic species. However, it has been found that, when an electrolysis cell is fed brine solution which results from a condensation polymer manufacturing process, such as a polycarbonate manufacturing process, the electrolysis cell separator still becomes plugged at an unexpectedly rapid rate even after the feed-brine solution has been subjected to primary brine treatment.
After careful experimentation it has been discovered that the cause of rapid separator plugging during electrolysis of such brine solution is the precipitation of transition metal species, primarily derived from residual iron, chromium, and nickel in the feed-brine, on the surface of and within the electrolysis cell separator. Analysis has revealed that there is still a very low concentration of transition metal species present in feed brine even after primary brine treatment because of the presence of a water-soluble chelating agent in the brine solution. The chelating agent apparently retains some fraction of the transition metal cations as a water-soluble complex so that these complexed cations are not precipitated as salts during primary brine treatment. The chelating agent is typically a sugar acid such as gluconate anion.
Gluconate anion is often added in the form of sodium gluconate in condensation polymer manufacturing processes to form water soluble complexes with a fraction of the multivalent transition metal cations such as iron (III), nickel (II), and chromium (III). Complexation beneficially hinders transition metal salts from precipitating in the manufacturing equipment and from contaminating the polymer product. With iron (III), for example, gluconate anion forms an iron-gluconate complex, thereby solubilizing iron in the brine solution so that the polymer product is produced substantially free of iron contamination. However, when the brine solution is being purified for recycle to an electrolysis cell, the fraction of a transition metal species such as iron (III) present as a gluconate complex remains strongly chelated, and the transition metal cation concentration in the brine solution may remain at an unacceptable level after brine purification processes such as-primary brine treatment and even after further treatment with cation exchange resins under essentially neutral or alkaline conditions. When brine containing transition metal-gluconate complexes such as the iron-gluconate complex enters an electrolysis cell containing a separator, the gluconate is substantially destroyed, and at least a portion of each transition metal, such as iron, precipitates on the surface of and inside the separator. The precipitated transition metal species gradually plug the separator and force lower production rates from the electrochemical cell and lead to premature separator replacement.
Methods for removing metal cations from an aqueous solution have been reported. Removal of multivalent metal cations from an aqueous solution using a chelating ion exchange resin is known. For example, Yokota et al. (U.S. Pat. No. 4,119,508) employ a chelating ion exchange resin to remove calcium and magnesium cations from a brine solution in the absence of a water-soluble metal chelating agent. Kelly (U.S. Pat. No. 4,450,057) utilizes AMBERLITE.RTM. IRC-718 (Rohm and Haas Company) to remove aluminum (III) from brine at pH 2 to 3 in the absence of a water-soluble metal chelating agent. Courduvelis et al. (U.S. Pat. No. 4,303,704) utilize AMBERLITE.RTM. IRC-718 resin at either acidic or alkaline pH to recover and reuse very high concentrations of copper or nickel ions from non-brine aqueous solutions derived from an electroless plating process and containing chelating agents such as alkanolamines. However, these methods do not address reducing the concentration of multivalent metal ions in brine solutions derived from a condensation polymer manufacturing process and containing a water-soluble metal chelating agent.
There is thus a need for a method which will substantially reduce the concentration of multivalent metal cations, such as transition metal cations, in brine solution derived from a condensation polymer manufacturing process and containing a water-soluble metal chelating agent. Such a method provides a means to retard plugging of an electrolysis cell separator, such as a diaphragm, by reducing the rate of precipitation of metal species on the surface of and inside the separator, thereby increasing the separator lifetime.